When they do their thing, female Chinese fruit bats add oral sex to get the males to prolong the act, scientists now find, suggesting the behavior confers evolutionary benefits.

Oral sex, or fellatio, is often used in human foreplay, the researchers noted, but rarely seen in other animals. As such, there have been few evolutionary reasons given for oral sex to date, and fellatio is largely thought confined to humans, although juvenile members of the chimpanzee-like bonobo do it for play.

Now scientists find the short-nosed fruit bat (Cynopterus sphinx) routinely engages in oral sex, the first time fellatio has been seen in adult animals other than humans. The researchers argue the act likely has evolutionary benefits.

Scientists at the Guangdong Entomological Institute in Guangzhou, China, and their colleagues investigated bats they captured at a nearby park. Although bats comprise the second largest order of mammals at more than 1,100 species, little is known about their mating habits because of their nocturnal lifestyle and their often-inaccessible roosts. The researchers were originally expecting to watch behaviors such as grooming or the construction of tents from Chinese fan-palm leaves.

"We did not expect fellatio in fruit bats at the beginning," said researcher Libiao Zhang, a biologist at the Guangdong Entomological Institute in Guangzhou, China. "We were also surprised at how often it occurred."

The investigators took digital videos at night of the bats having sex, with males keeping a tight grip on the females from behind by holding thumbs on the females' wings clamping down with their mouths on the females' necks.

Intriguingly, the female lowered her head to lick the male penis during 14 of 20 copulations recorded on video. The licking typically went on for some 19 seconds, or roughly one-twelfth the average time of copulation. The male never withdrew from the female when she performed fellatio.

The researchers found the longer the fellatio went on, the longer the bats often had sex, with each second of licking adding roughly six extra seconds of copulation. The bats spent almost twice as long copulating when oral sex was involved than when not.

"It was difficult to provide some hypotheses for the function of the fellatio behavior," Zhang said. "We held many meetings to discuss the functions."

Prolonging sex might increase the chances of getting pregnant by aiding transport of sperm and stimulating female glandular secretions, they suggested. It might even help prevent sexually transmitted diseases from the male to the female by killing germs with saliva, so while the male may enjoy the act, the female ultimately benefits.

The scientists detailed their findings online Oct. 28 in the journal PLoS ONE.